This invention relates to electroluminescent devices and more particularly to electroluminescent devices of the thin film type.
Luminescence is a phenomenon which occurs in gases, liquids and solids wherein exciting energy which may come from light, an electron beam, or passage of an electric current therethrough raises atoms or molecules to higher energy levels and wherein emission of radiation thereafter occurs as particles fall back to lower energy levels. Since atoms and molecules can have only certain discrete energies, these processes can occur only in jumps. For each jump, the emitted radiation has a frequency given by the energy jump divided by Planck's constant.
Intrinsic electroluminescence was discovered by G. Destriau in 1936 by exposing phosphor materials to high electric fields. It was not until some time later that a practical device was developed consisting of a dispersion type of electroluminescent structure. Such a structure is comprised of activated phosphor powder dispersed in a layer of insulating substance. Such a device exhibits very poor brightness, efficiency and a relatively short useful life. Although recent advances in electronics and material science has enhanced the development of such devices, the advent of thin film technology has resulted in the development of a significant improvement comprising a double insulating layer structure. There a vacuum deposited thin phosphor film replaces the dispersion layer structure. The problems related to short life and low brightness, the major obstacles that hindered practical application up to that time have been substantially resolved thereby. The vacuum deposited thin film electroluminescent device with a double insulating layer structure, while exhibiting a relatively higher brightness and longer life, was found to have an inherent weakness in that any breakdown in the insulating layer adjacent the outer electrode was followed by a catastrophic effect of the electrode due to the localized high current conductivity which occurred initially at the point of breakdown and which subsequently spread until ultimate failure of the device occurred.
Accordingly, it is an object of the present invention to provide an improvement in electroluminescent devices.
It is a further object of the present invention to provide an improvement in the construction of thin film electroluminescent devices.
Still another object of the present invention is to provide an improvement in the structure of thin film electroluminescent devices which exhibit an enhanced resistance to failure.